34
Mental calculation methods
Textbook: Part 1, lesson 33, pp. 70–71
GoalMental calculation methods; differences that do not change; Roman numerals.
New words
mental calculation · og‘zaki hisoblashpattern · qonuniyatRoman numerals · Rim raqamlariparcel · jo‘natma
Explanation
If the minuend and the subtrahend both grow by the same number, the difference does not change: 18 − 2 = 19 − 3 = 20 − 4 = 16. We add in a convenient order. Roman numerals: I = 1, V = 5, X = 10; IV = 4, VI = 6.
Worked examples
28 − 16 = 12; 38 − 26 = 12; 48 − 36 = 12 – the difference is the same.
26 + 12 + 14 = (26 + 14) + 12 = 40 + 12 = 52.
Class activity
“Pattern detectives”: 18 − 2, 19 − 3, 20 − 4 ... are written on the board. Children find the next answer without calculating and explain why.
Practice
1
38 − 26 = ?
12
2
26 + 12 + 14 = ?
52
3
XII + VIII = ? (in Arabic numerals)
20
4
Today 35 parcels were delivered and yesterday 8 fewer. How many parcels in the two days?
62